1. Field of the Invention
The present invention relates to an optical data recording and reproducing apparatus and method, and more particularly, to an optical data recording and reproducing apparatus and method having a stationary recording medium with a spherical recording surface.
2. Description of the Prior Art
Conventionally, an optical disk drive irradiates a fiat recording surface of a disk type recording medium with a laser beam through an object lens, to focus on a recording surface for recording and reproducing data. The disk type recording medium is mounted on a spindle and rotates at a predetermined number of rotations. Recordation and reproduction of data are carried out by focusing the laser beam on the recording surface of the disk type recording medium with an optical head and fixing the beam to the predetermined track. A servo technique for focusing a laser beam is called "focusing," and fixing a beam to a track is called "tracking."
An optical disk drive can roughly be classified into three types, depending on a material of the recording layer coating the recording surface. A first type is called a read-only type which is mainly used as a video disk, compact disk (CD) and compact disk ROM (CD-ROM). In this type of optical disk drive, recording data is realized by mechanically or chemically providing permanent pits to the recording surface at the time of producing a recording medium and then forming an aluminum reflecting film on such projections and recesses. Meanwhile, reading and reproduction of data is realized by focusing a laser beam on this recording surface and sensing intensity or interference of the reflected beams.
A second type is called a write-once type which allows users to freely write data once, but does not allow users to write more data on the same recording area. Namely, an optical disk drive of this type allows only one writing of data to the recording area. Recording of data is generally realized by boring pits on the recording surface or changing the optical property to locally change a reflectivity of the recording surface by irradiating the recording surface with an intensified laser beam. The non-reversible change of such optical property allows only one recording of data. Reading and reproduction of data is realized, like the read-only type, by focusing the laser beam on such a recording surface and then sensing intensity or interference of the reflected light beam. Reproduction of data can be repeated as many times as desired.
FIG. 1(a) (PRIOR ART) illustrates a recording surface of the write-once type optical disk drive. Tracks TRK are formed on a substrate 31 of the recording surface and grooves TRG are also formed between the tracks. The groove TRG is generally formed in the shape of the letter V having a width of 0.4 .mu.m, depth of 70 nm and interval to the adjacent groove of about 1.6 .mu.m. In the track TRK, pits 32 are bored by an intensified laser beam depending on the recording data bits "0" and "1" Reproduction of data is carried out as follows. When a track TRK is irradiated with a laser beam RB through an object lens 4251 as illustrated in FIG. 1(b) (PRIOR ART), the beam is almost reflected at the area where a pit 32 is not provided and returns to the object lens 4251. But at the area where a pit 32 is bored, the laser beam is diffracted by the pit and the reflected laser beam goes out of the sight of the object lens 4251 and only a part of the beam returns to the object lens 4251. Therefore, the stored data can be reproduced by sensing the returning laser beam with a photodiode.
A third type of an optical disk drive is a rewritable type which allows repeated recording of data to the target recording area. For the rewriting of data, it is an essential condition that a property of the recording surface can be changed reversibly with irradiation of a laser beam. Various methods for reversibly changing the property of the recording surface have already been proposed and a magneto-optical recording system and a phase change recording system are typical methods. In the magneto-optical recording system, recording is carried out by magnetizing the recording surface, consisting of a magnetic film, in one direction (erased condition) or locally inverting the magnetizing direction (recorded condition) with an external bias magnetic field and laser beam. The laser beam works to assist the inversion of the magnetizing direction and only the areas which are locally heated by irradiation of an intensified laser beam are inversed. Reading and reproduction of data can be conducted by irradiation of a weak laser beam. In this case, a change of a polarizing surface of the laser beam due to the magnetizing direction is detected with a detector by utilizing the Kerr effect and then the change is converted to an intensity of the laser beam.
FIGS. 2(a)-(g) (PRIOR ART) are diagrams for explaining the recordation and reproduction principle of the magneto-optical recording system. As illustrated in FIG. 2(a), a magnetic field is applied with a recording coil C in the upward direction to a magnetic film M, when it is magnetized in the downward direction. Magnetic film M may be an alloy thin film, etc. of a rare earth metal and a transition metal such as MnBi thin film or TbFeCo deposited on a recording substrate. As illustrated in FIG. 2(b), when the magnetic film M is then irradiated with a laser beam RB through an object lens OL only at an area where the magnetizing direction is to be inverted, the magnetized direction of only such area is inverted to an upward direction, permitting the recording of data.
For the reproduction of recorded data, when the magnetic film M is irradiated with the laser beam RB having the polarizing surface in the direction of the y axis, as illustrated in FIG. 2(c), the reflected laser beam RB0, having a polarizing surface which is rotated by .THETA. in the clockwise direction as illustrated in FIG. 2(d), can be obtained by means of the Kerr effect in the area where the magnetizing direction is set downward, as illustrated in FIG. 2(e). Moreover, the reflected laser beam RB1, having a polarizing surface which is rotated by .THETA. in the clockwise direction as illustrated in FIG. 2(f), can be obtained at the area where the magnetizing direction is set upward, as illustrated in FIG. 2(g). Therefore, data can be reproduced by sensing the polarizing condition of the reflected laser beam. Meanwhile, a phase change recording system utilizes a change of the optical property of the recording surface. This is a change of reflectivity of the recording surface when the recording surface is quickly heated or cooled and gradually heated or cooled with the laser beam. This phase change recording system has a merit that a bias magnet which has been required in the magneto-optical recording system is no longer required.
In any type of optical disk drive, the focusing and tracking are very important because a disk type recording medium is rotated at a high speed and data is read and reproduced during rotation of the recording medium, as explained above. The disk type recording medium shows, during its rotation, a deflection of a surface as large as 0.2.about.0.3 mm. Therefore, if the object lens of an optical head is fixed, the focal point deviates from the recording surface on the recording medium, making it difficult to read and reproduce data. In the case of the write-once type or rewritable type, recording is also difficult. In order to always keep constant the distance between the lens and the recording surface, the object lens is generally moved perpendicular to the recording surface and is caused to follow the surface deflection of the recording medium. This may be realized by a Knife-edge method, an Astigmatic method or a Foucalt method.
Moreover, when the disk recording medium is set to the spindle of the optical disk drive for the rotation, an eccentricity of about 50 .mu.m is generated between the laser beam emitted from the object lens and a track on the recording surface. This is due to axial deflection of the spindle and eccentricity of the center hole of the recording medium. For accurate recordation and reproduction of data, it is essential that the laser beam can accurately follow the tracks. For this purpose, a push-pull method and a three-beam method have been proposed.
As described above, an optical disk drive of the prior art is required to provide a tracking and compensating device in order to eliminate deflection due to surface deflection, axial deflection of a disk type recording medium during rotation thereof, and eccentricity of the center hole of the recording medium. Therefore, accuracy of the tracking and compensating device causes a certain restriction on the high density and high rotating speed of the optical disk drive. Moreover, a reproduction error is inevitably increased and an intensive error correcting technique is also required for eliminating the influence of such reproduction error. In addition, a large number of redundancy bits are necessary for error correction, thereby limiting a substantially higher density.
In view of solving the foregoing problems, U.S. Pat. No. 4,995,025 has been proposed. U.S. Pat. No. 4,995,025 discloses an optical disk drive where a plate type recording medium having a curved surface is stationarily fixed and an optical head swings around a pivot shaft. However, in this optical disk drive, one end of the optical head is located at the pivot shaft, while the other end is approximated at the recording medium surface. Therefore, the gravity center of the optical head is isolated from the pivot shaft. Moreover, acceleration and deceleration periods are necessary for reciprocally swinging the optical head. Accordingly, a large force is required for swinging this optical head at a high speed. Actually, it is impossible to move such an optical head at a high speed.